In a typical cellular wireless communication system, an area is divided geographically into a number of cells and cell sectors, each defined by a radio frequency (RF) radiation pattern from a respective base station antenna. The base station antennae in the cells may then be coupled with a base station controller, which may then be coupled with a switch or gateway that provides connectivity with a transport network such as the public switched telephone network (PSTN) or the Internet.
When a mobile station, such as a cellular telephone, pager, or wirelessly-equipped computer, is positioned in a cell, the mobile station communicates via an RF air interface with the base station antennae of a cell. Consequently, a communication path can be established between the mobile station and the transport network, via the air interface, the base station, the base station controller, and the switch or gateway.
Further, in some wireless communication systems, multiple base stations are connected with a common base station controller, and multiple base stations are connected with a common switch or gateway. Each base station controller may then manage air interface resources for multiple wireless coverage areas (e.g., multiple cells and sectors), by performing functions such as assigning air interface traffic channels for use by mobile stations in the coverage areas and orchestrating handoff of calls between coverage areas. And the switch and/or gateway, in turn, may control one or more base station controllers and generally control wireless communications, by performing functions such receiving and processing call requests, instructing base station controllers when to assign traffic channels, paging mobile stations, and managing handoff of calls between base station controllers.
In general, air interface communications in each sector (or other such coverage area) of a cellular wireless communication system can be encoded or carried in a manner that distinguishes the communications in that sector from communications in adjacent sectors. For example, in a Code Division Multiple Access (CDMA) system, each sector has a respective pseudo-random noise offset or “PN offset” that is used to encode or modulate air interface communications in the sector distinctly from those in adjacent sectors. Analogously, in other air interface protocols, communications in one sector may be distinguished from those in other sectors by frequency, time, and/or various other parameters.
Furthermore, each sector generally has a limited set of resources that can be allocated for use to serve mobile stations in the sector. By way of example, each sector may define an air interface “access channel” on which mobile stations can send “access probes” seeking to originate calls (e.g., voice calls, data sessions, and/or other “calls”) or seeking to register their presence in the sector. The access channel may itself have limited capacity. (Further, if multiple access channels are provided, they may cooperatively have limited capacity.) For instance, the access channel may define timeslots in which mobile stations can send access probes and may thus have a limited number of such timeslots. If numerous mobile stations are sending access probes in the same sector around the same time, the access channel of the sector can become congested and can ultimately reach a point where any further attempts to send access probes would result in “access probe collisions” and thus call setup failures (blocked calls) or other registration failure.
As another example, each sector may define an air interface “paging channel” on which the serving base station can send access probe acknowledgements and traffic channel assignment messages to served mobile stations. And the paging channel may similarly have limited capacity. (Further, if multiple access channels are provided, they may cooperatively have limited capacity.) For instance, the paging channel may similarly define timeslots in which the base station can send various messages to particular mobile stations. If the base station has numerous such messages to send, however, the paging channel can become congested and can thereby delay call setup or the like.
As yet another example, each sector may have a limited amount of transmission power for base station transmissions to served mobile stations. That transmission power may need to be divvied among numerous base station transmissions, such as transmissions to specific mobile stations and broadcast transmissions to mobile stations generally. At some point, if there is too much demand for base station transmissions, the power level allocated to particular transmissions may decrease to a point that the quality of the transmissions may suffer.
As still another example, each sector may have a limited number of traffic channels that its serving base station can assign at any given time (e.g., for concurrent use by numerous mobile stations, or for other use). In CDMA, for instance, each traffic channel may be defined by encoding with a particular “Walsh code,” yet the sector may have a limited pool of such Walsh codes. Consequently, if more than that number of traffic channels are needed at a given time, the base station would need to reject additional requests for traffic channel assignment, thus blocking calls. Alternatively, in time division multiplex systems, such as TDMA or 1×EV-DO (e.g., the 1×EV-DO forward link for instance), traffic channels may be defined through interleaved timeslots on the air interface. In that case, if more than a threshold extent of air interface communication occurs at once, the base station may be unable to serve additional communications, due to the absence of available timeslots. As a result, communications may be blocked or degraded.
And as yet another example, each sector may have a limited supply of hardware addresses, such as Medium Access Control identifiers (MAC IDs) that its serving base station may assign for use to identify mobile stations operating in the sector. This is typically the case in systems operating according to the 1×EV-DO protocol for instance. If more than a threshold number of mobile stations are operating in the sector at once, the base station may exhaust its supply of MAC IDs and may then be unable to serve additional mobile stations that seek call initiation. Consequently, when additional mobile stations request call initiation, the base station may need to reject their requests, again resulting in blocked calls.
In a typical cellular wireless communication system, the base station of a given sector will broadcast various overhead messages to provide the mobile stations operating in the sector with information that will enable the mobile stations to communicate with the base station. One such message may be an “access parameters message” or the like, which provides information about access channel communication, such as (i) the power level at which a mobile station should transmit an initial access probe seeking to originate a call, (ii) the extent to which the mobile station should increase the transmit power for each successive access probe attempt until the mobile station receives an access probe acknowledgement from the base station, (iii) the length or type of data the mobile station is to include in its access probes, and (iv) the length of each access probe period, among other information.
Another such message may be a “sector parameters message” or “system parameters message” or the like, which provides other overhead information for the sector, such as an identifier of the sector and the geographic location of the serving base station. Of particular interest, the sector parameters message may also provide a “neighbor list” for the sector, which is a list of adjacent sectors, to which the mobile station may hand off. In practice, a base station will broadcast a pilot signal on each of its sectors, to allow mobile stations to detect the presence of the sector. When a mobile station is engaged in a call on a given sector, the mobile station may then scan for pilot signals of the sectors listed in that sector's neighbor list, in search of a strong enough sector to justify handoff. If the mobile station identifies such a sector, the mobile station may then hand off to that other sector.
When a mobile station is in an idle mode, i.e., not currently engaged in an active call, the mobile station may also monitor the pilot signals of various sectors, to determine a best (e.g., strongest) sector in which the mobile station should operate. Once the mobile station identifies such a sector, the mobile station may then read the sector parameters message of the sector to learn more about the sector. And the mobile station may then periodically scan the paging channel in that sector, in search of page messages that would alert the mobile station of incoming calls. Further, when the mobile station then seeks to initiate a call the mobile station would send an access probe on the access channel of that sector. As noted above, the base station may then send an access probe acknowledgement. In turn, the network (e.g., base station controller) may then assign an air interface traffic channel for the call, and the base station may send a traffic channel assignment message via the paging channel to the mobile station. The mobile station may then use the assigned traffic channel to engage in the requested call.
When a switch in a cellular wireless communication system seeks to page a mobile station (e.g., for an incoming call or for some other reason), the switch can send the page message to numerous base stations in the switch's coverage area, with the hope that when the base stations broadcast the page message, the mobile station will receive the page message in one of the associated sectors and will respond. Given the scarcity of paging channel resources, however, most modern cellular networks are instead arranged to engage in a more targeted paging process known as “zone based paging.”
With zone based paging, a cellular network is divided into paging zones, each with a respective zone ID, and paging is performed on a zone-basis. To facilitate this, each base station in the system may broadcast as one of its overhead parameters the zone ID for the zone in which the base station is located. Mobile stations operating in the network may then programmatically monitor the zone IDs indicated in the overhead messages and may automatically register with the network when they detect that they have moved into a new zone, or for other reasons. To register with the network, a mobile station may send a registration message via the access channel in its current sector, and a switch in the network would note the mobile station's registration and convey an indication of the registration to a home location register for later reference.
With this process, the registration records thereby maintained by switches and/or home location registers will indicate the paging zone in which each mobile station last registered. When a switch seeks to page a mobile station, the switch may then efficiently send the page message to just those base stations that are within the zone of the mobile station's last registration, as it is likely that the mobile station is in that zone. Further, the switch may send the page message to the base stations in zones adjacent to the mobile station's zone of last registration, to cover the possibility that the mobile station has moved to a new zone but has not yet registered its presence in the new zone. Once the designated base stations transmit the page message, if the mobile station does not respond to the page, the switch may then broaden the scope of the page, by sending the page message to a wider range of paging zones and perhaps ultimately to all base stations in the switch's serving area.